The bolt nose recess dimension won't tell you much. The distance the cartridge sticks out of the barrel or chamber is dependent on the action used. Some sit in a recess so the cartridge is near flush. Head space is still measured with gauges and a depth gauge. With a cone nose bolt you measure from the shoulder of the barrel to the bolt face and compare the shoulder to Go-Gauge to determine headspace.

What are you working on?

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That is what I was trying to figure out. How much the cartridge stuck out the back of the barrel on some different actions with the .50 BMG. I thought no matter the action or depth of the bolt nose the space between the bolt and the back of the barrel is probably kept within .01? Therefor the bolt face depth dimension would get me close.